ALSA: au88x0: Give comment for vortex_wtdma_bufshift() issue
The check of the return value from vortex_wtdma_bufshft() in
vortex_interrupt() is useless as it executes no code. Actually the
return value is intentionally ignored because the delta calculation
for wavetable doesn't work always correctly. For avoiding the
confusion, a comment is added and the superfluous if () is removed.